1. Field of the Invention
The present invention relates to a circuit board which has a circuit on an insulating substrate, and a method for producing the same. The present invention relates to a circuit board which is preferably used, for example, as a circuit board for cooling an electronic circuit chip composed of a semiconductor or the like, and a method for producing the same.
2. Description of the Related Art
It is important for a general circuit board for mounting thereon a semiconductor device to efficiently transmit the heat generated by the semiconductor device to the outside. In other words, the heat is a great enemy for the semiconductor device. It is necessary that the internal temperature does not exceed the maximum permissible joining temperature. In a semiconductor device such as a power transistor or a semiconductor rectifying device, the electric power consumption per operation area is large. Therefore, the generated amount of heat is released insufficiently with only an amount of heat released from the case (package) and the lead of the semiconductor device. The internal temperature of the device may be raised, resulting in thermal destruction.
A semiconductor device which carries CPU suffers the same problem as described above. The generated amount of heat is increased as the clock frequency is improved. Therefore, the thermal design in consideration of the heat release is important.
According to the thermal design of preventing the thermal destruction, the circuit board design and the mounting design are performed for securing a heat sink having a large heat release area to the case (package) of the semiconductor device.
Explanation will now be made with reference to FIG. 36 for a conventional circuit board 200 to which a heat-control measure is applied (see, for example, Japanese Laid-Open Patent Publication No. 11-307696).
As shown in FIG. 36, the circuit board 200 comprises a metal base plate 202 for releasing the heat generated by a semiconductor chip, a ceramic plate 206 for insulating the semiconductor chip 204 from the metal base plate 202, a circuit 210 of a metal plate 224 disposed on the upper surface of the ceramic plate 206 with a brazing member 208 interposed therebetween, a lower electrode plate 214 disposed on the lower surface of the ceramic plate 206 with a brazing member 212 interposed therebetween, a metal spacer 216 for widening the distance between the metal base plate 202 and the ceramic plate 206, a brazing member 218 for securing the metal spacer 216 to the metal base plate 202, a solder layer 220 for securing the semiconductor chip 204 onto a circuit 210, and a solder layer 222 for securing the lower electrode plate 214 onto the metal spacer 216.
In the conventional circuit board 200 shown in FIG. 36, when the circuit 210 is formed on the ceramic plate 206, the metal plate 224 is firstly joined onto the ceramic plate 206 using the brazing member 208. Next, the metal plate 224 is selectively etched to form the circuit 210 having a predetermined circuit pattern. The technique for forming the circuit 210 by etching the metal plate 224 is disclosed, for example, in Japanese Laid-Open Patent Publication Nos. 8-97554, 9-181423, and 7-235750.
A brazing member containing an active metal is used as the brazing member 208 for joining the metal plate 224 onto the ceramic plate 206. In this case, it is possible to improve the joining strength between the ceramic plate 206 and the circuit 210.
However, when the ceramic plate 206 and the metal plate 224 are joined to one another using the brazing member 208 as shown in FIG. 36, a conductive reactive layer 226 is formed between the ceramic plate 206 and the metal plate 224 as shown in FIG. 37. The conductive reactive layer 226 cannot be removed by etching the metal plate 224, i.e., by the etching the metal plate 224 using an aqueous solution of ferric chloride or an aqueous solution of cupric chloride normally used for etching copper. The conductive reactive layer 226 consequently remains on the ceramic plate 206. If the conductive reactive layer 226 remains on the ceramic plate 206, the circuit 210 may be short-circuited.
In order to solve the problem, a method has been suggested, in which any unnecessary brazing matter (etching residue) containing a nitride layer of active metal, which remains after the circuit etching with the ferric chloride solution or the cupric chloride solution, is removed by performing another acid treatment after the etching step.
For example, Japanese Patent No. 2594475 discloses a method for removing the unnecessary brazing matter with hydrofluoric acid singly or with mixed acid of inorganic acid and hydrofluoric acid. Japanese Laid-Open Patent Publication No. 4-322491 discloses a method for removing the unnecessary brazing matter with ammonium halide. Japanese Laid-Open Patent Publication No. 5-13920 discloses a method for removing the unnecessary brazing matter with inorganic acid and hydrogen peroxide after treating the unnecessary brazing matter with hydrogen halide or ammonium halide.
Japanese Laid-Open Patent Publication No. 7-235750 discloses a method for removing the unnecessary brazing matter with a solution containing fluorine compound and hydrogen peroxide but containing no inorganic acid. Japanese Laid-Open Patent Publication No. 10-154866 discloses a method for removing the unnecessary brazing matter. The method comprises the steps of treating the unnecessary brazing matter with ammonium fluoride and hydrogen peroxide and treating the unnecessary brazing matter with a solution of alkaline and hydrogen peroxide.
However, according to these techniques, it is necessary to consider the safety of operation, because the hydrofluoric acid-based solution is used. Further, it is difficult to manage the steps, and it is impossible to completely remove the nitride layer of active metal.
It is assumed to use a technique for removing the unnecessary brazing matter by the honing laser machining as a technique for mechanically removing the unnecessary brazing matter (see, for example, Japanese Laid-Open Patent Publication No. 7-99380). However, the large apparatus may be constructed and the production cost is also large.